METO 637

LESSON 3

Photochemical Change

• A quantum of radiative energy is called a photon, and is given the symbol hHence in a chemical equation we write:

O3 + h→O2 + O

• The energy of a photon in terms of its wavelength is E=119625/kJ mol-1 or 1239.8/ in eV

• To get enough energy to break up a molecule (dissociation) the wavelength must be in or below the ultraviolet. Thus dissociation typically occurs as the result of electronic transitions

• Small, light chemical species generally have electronic transitions at wavelengths shorter than those for more complex compounds, e.g. <200 nm for O2,.

Photochemical Change• Atmospheres tend to act as filters cutting out short wavelength

radiation, since the absorptions of their major constituents are generally strong at the short wavelengths.

• As a result, photochemically active radiation that penetrates into an atmosphere is of longer wavelengths, and the chemistry is characterized by lower energies. For example, the dissociation of molecular oxygen, the ultimate source of ozone in the stratosphere, is limited to altitudes above 30 km.

• Absorption of a photon of photochemically active radiation leads to electronic excitation, represented as

AB + h →AB*

• If the excited molecule then breaks apart, the quantum yield of such a reaction is defined as the number of reactant molecules decomposed for each quantum of radiation absorbed

Absorption in molecular lines and bands

• Molecules have three types of energy levels - electronic, vibrational, and rotational

• Transitions between electronic levels occur mainly in the ultraviolet

• Transitions between vibrational levels - visible/near IR

• Transitions between rotational levels - far IR/ mm wave region

• O2 and N2 have essentially no absorption in the IR• 4 most important IR absorbers H2O, CO2, O3, CH4

Schematic of vibrational levels

Photochemical Change

Photodissociation

• Two main mechanisms are recognized for dissociation, optical dissociation and pre-dissociation. These processes will be illustrated in the O2 and O3 molecules.

• Optical dissociation occurs within the electronic state to which the dissociation first occurs. The absorption spectrum leading to dissociation is a continuum.

• At some longer wavelength the spectrum shows vibrational bands. The bands get closer together as the limit is approached – the restoring force for the vibration gets weaker.

•The absorption from the ‘X’ to the ‘B’ state in O2 , is an example.

Photodissociation

Photodissociation

Photodissociation

Photodissociation• Note that when the B state dissociates, one of the two atomic

fragments is excited. One atom is left in the ground state (3P) and the other in an excited state (1D).

• Some fragmentation occurs in the B→X (Schumann-Runge) system before the dissociation limit. This occurs because a repulsive state crosses the B electronic state and a radiationless transition takes place. The repulsive state is unstable and dissociation takes place. Note that both atomic fragments are 3P.

• Although molecular oxygen has many electronic states, not all of the possible transitions between the states are allowed. The magnitude of the photon energy is not the only criteria

• Consideration of things such as the need to conserve quantum spin and orbital angular momentum indicate if the transition is possible.

Predissociation

• Predissociation arises when two electronic states cross.

• Usually one electronic state is stable - has a well defined potential diagram. The other state is typically a repulsive state.

• The transition is to the stable state, and then moves across to the repulsive state, when dissociation occurs.

Franck-Condon principle

• The time for a transition is extremely small, and in this time the atoms within a molecule can be assumed not to move.

• Franck and Condon therefore postulated that on a potential energy diagram the most likely transitions would be vertical transitions

Franck-Condon principle

Predissociation

Photodissociation• Let us take the reaction

O3 + h→ O2 + O• The O2 molecule and atom can be left in several states if we

consider energy alone, because any ‘extra’ energy can be used for kinetic energy of the products.

• For wavelengths about 310 nm or less, spin conservation allows the transition

O3 + h→ O2(1g) + O(1D)• The O(1D) atom formed in this reaction plays a major role in

atmospheric chemistry, for example

O(1D) + H2O → OH + OH• OH, the hydroxyl radical, can break down hydrocarbons.

Wavelength Threshold for Dissociation of Ozone

Quantum yield for Ozone

Quantum yield for Ozone

• Note that the onset of dissociation is not abrupt. • The shape of the curve can be explained if the

internal energy of the molecule (vibration and rotation) can be added to the photon energy to induce transitions.

• Transitions from vibrationally excited states can be important in the atmosphere.

• The solar spectrum shows a rapid increase above 310 nm, so any extension of the absorption cross section above this limit can lead to a significant increase in say the quantum yield of O1D